The method described herein relates generally to in-situ magnetization and degaussing of generator rotors. More specifically, the apparatus relates to in-situ magnetization and degaussing of rotors incorporating permanent magnets.
Most synchronous machines include either permanent magnets or excitation windings (fed by a regulated source) to provide MMF (magnetomotive force) that provides the magnetic flux for machine operation. The permanent magnets or excitation windings can be situated on either the rotor or the stator. For rotor-based embodiments, excitation power is brought through either a set of collectors (slip rings) or a brushless system that uses a small (“inside-out”) synchronous machine with a stator excitation source.
Permanent magnet machines are not easily regulated. Operation of permanent magnet machines in constant power mode can be a problem because low power factor operation is forced through flux weakening methods to reduce voltage at light loads or to minimize inverter ratings. As a result, the machine terminal voltage becomes load dependent, efficiency suffers, and, at partial loads, the magnetic field source is underutilized.
Machines wound with excitation windings (“wound field machines”) can be regulated over a wide range of loads, but wound field machines experience winding losses that decrease machine efficiency. Additionally, windings and excitation sources for wound field machines are sized to support the maximum requirements and thus are often expensive and under-utilized.
Some known generators combine the advantages of the excitation control of wound field machines and the advantages of higher efficiency of permanent magnet machines. A hybrid machine comprises a cylindrical element having slots, excitation windings situated in at least some of the slots, and permanent magnets situated at the poles. For example, U.S. Pat. No. 6,509,664 describes a hybrid synchronous machine with a field/rotor having excitation windings is some of the slots and permanent magnets at the poles, and this patent is incorporated herein by reference.